Prepare for sync MVP

[ErikBjare]

• Added support for specifying filename in PeeweeStorage constructor.
• Added JSON schemas for Bucket and Export.

Required by: ActivityWatch/aw-server#50

[codecov]

Codecov Report

Merging #69 into master will decrease coverage by 0.06%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   94.52%   94.46%   -0.07%     
==========================================
  Files          31       31              
  Lines        1352     1354       +2     
  Branches      218      219       +1     
==========================================
+ Hits         1278     1279       +1     
  Misses         37       37              
- Partials       37       38       +1

Impacted Files	Coverage Δ
aw_core/__init__.py	100% <100%> (ø)	⬆️
aw_datastore/storages/peewee.py	95.74% <87.5%> (-0.69%)	⬇️

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